Physical properties of food are characteristic values important to controlling food qualities in processing, distribution and consumption. In particular if a viscosity can be easily measured, it is possible to know not only processing aptitudes for cooking, filling and so on but also textures and easiness in handling, as well as comparison with another food can be facilitated.
There are various types of viscosity measuring apparatuses, and the types are roughly classified into a rotation type and a translation type.
The rotation type viscosity measuring apparatus is advantageous in providing a simple measurement at low cost, and is suited for measuring a uniform sample having a low viscosity. However, when a sample having a high viscosity such as a gel is measured by the rotation-type viscosity measuring apparatus, an internal structure of the sample varies because of a “shear deformation” or vibrations give thereto until a measurement value becomes stable. Thus, there is a problem in that a viscosity of the sample is measured to be lower than an actual viscosity.
On the other hand, the translation-type viscosity measuring apparatus is advantageous in having a simple apparatus structure, without any rotating and driving unit. There are a translation-type viscosity measuring apparatus of a parallel plate type and a translation type viscosity measuring apparatus of a concentric cylinder type. Non-Patent Documents 1 to 4 disclose a viscosity measuring method using a viscosity measuring apparatus of a concentric cylinder type.
The method (referred to also as back extrusion (BE) method) disclosed in the Non-Patent Documents 1 to 3 is a typical translation type viscosity measuring method. In this viscosity measuring method, a cylindrical plunger is pushed into a sample contained in a cylindrical container from outside the sample, the sample is made to flow upward in an annular gap part between the container and the plunger, and a viscosity is calculated from a stress-time curve applied to the plunger. Although this method can analyze from a Newtonian fluid up to a Herschel-Bulkley fluid, it is necessary to increase a deformation degree applied to the sample in order to obtain a steady flow in the annular part, which deformation for measurement destroys the structure of the sample. Thus, this method cannot measure the same sample consecutively. In addition, when a sample has a high viscosity, it is necessary to carefully remove the sample adhering to the container and the plunger after measurement. Thus, the operation is complicated and a longer time is needed. Moreover, since the method is somewhat inferior in measurement precision, the method is not so generally prevalent.
On the other hand, the method (referred to also as short back extrusion (SBE) method) disclosed in the Non-Patent Document 4 by the present inventors is a method wherein a cylindrical plunger is previously immersed by a predetermined depth into a sample contained in a cylindrical container, the plunger is further pushed thereinto from the original position by a slight distance to generate a steady flow in an annular part, and a viscosity is measured from a stress-time curve applied to the plunger. In this method, the annular part between the plunger and the container has been already filled with the sample before the plunger is pushed thereinto, which is different from the BE method. Thus, even if a movement distance of the plunger is short, a steady flow can be generated. Thus, since an amount of the sample adhering to the plunger and the container is small, consecutive measurement is enabled.
The present inventors have already proposed a method of analyzing a Newtonian fluid and a power-law fluid (see Patent Documents 1 and 2). This method can perform viscosity measurement that is more precise than the BE method. However, a method of analyzing Herschel-Bulkley fluid by the SBE method is not known.
By the way, a non-Newtonian fluid is a fluid whose viscosity is dependent on a “shear rate”. A viscosity of the non-Newtonian fluid is represented as an apparent viscosity obtained by dividing a “sheer stress” by a “shear rate”. In addition, the power-law fluid is a non-Newtonian fluid whose minimum value of the shear stress (referred to as yield stress), which is necessary to start flow, is zero. The Herschel Bulkley fluid is a non-Newtonian fluid whose yield stress is larger than zero.